Quoted Text
...There is a box on the rear plate over the transmission hatch with a pipe sticking out of it. Do you know if that is a smoke generator?...
Dave,
The box is not a smoke generator, but a "Печь Танковая Типовая"- Standard Tank Heater(STH):

Its purpose was to keep the engine-transmission compartment warm during cold nights.
It burned wood or diesel fuel and saw a widespread use in the winter of 1942/1943.
The STH was made obsolete by the engine preheater, installed on all V-2 tank engines from 1943 onwards.
Standard procedure for use of STH- crew digs a shallow trench, the STH is placed in it and lighted, the SPG/Tank is parked over the trench so that the engine-transmission comptartment is above the STH, then the SPG/Tank is covered by a canvas.Two examples:


STH fixtures were present on all SU-122 Mk.2(like Zvezda's kit), but obviously, the STH was only issued and used during cold seasons.
I would do the box rusty and sooty
HTH